In electrolytic cells for the electrolysis of aqueous alkali metal chloride solutions there may be used as cathode, at least as a substrate metal, iron or stainless steel or the like. Such a substrate might contain an active, metallic surface coating. It has, however, been observed that the cathodes can be susceptible to corrosion. It has been found that the use of certain additives will help in retarding this corrosion. Thus, in U.S. Pat, No. 4,379,035, where it is shown to use cathode coatings of porous or activated nickel on a steel substrate, there is taught the addition of small amounts of alkali metal benzoate and alkali metal nitrite to the catholyte compartment of the cell. Such addition can be made directly to the catholyte liquor contained in this compartment. The patent teaches that this addition will retard corrosion of the cathode and, thus, ostensibly aid in extending cathode life.
As discussed in U.S. Pat. No. 4,358,353, cathodes as used in electrolytic cells may be partitioned by a separator such as an asbestos diaphragm or synthetic microporous separator. During operation it can be expected that the cell will be subjected to current interruption. This might be caused simply by routine cell maintenance. According to the teachings in this U.S. Pat. No. 4,358,353, such current interruption can result in the release of sodium hypochlorite in the catholyte which can have an adverse effect on a coating of the metal cathode. It can also be expected that such phenomenon will occur with uncoated cathodes. To improve cathode durability for coated cathodes in view of this, this patent teaches the addition to the catholyte of small amounts of reducing agent which reacts with the sodium hypochlorite to prevent the oxidation, and retard dissolution of the transition metal in the cathode coating. Such patent more particularly details the addition of an alkali metal sulfite or urea.
A similar observation of cathode degradation during cell shutdown, particularly repeated shutdown, is made in U.S. Pat. No. 4,539,083. The solution proposed by this teaching for extending cathode life is also to add a reducing agent to the cathode compartment of the electrolytic cell. The additives proposed by this teaching are agents such as sulfites and phosphites.
As technology improves whereby electrolytic cells are maintained in operation for longer periods of time, current interruptions can become more and more of a factor in degradation of cell components. Additionally, such extended operation for the cells may also create the problem of enhancing the introduction of impurities into cell products. Thus, as cell operations become more extended, it becomes more challenging to provide consistent, high quality product for the life of the cell as well as extended life for all cell components.